The present invention relates to substantially inorganic intumescent fire protective sheaths useful for the protection of electrical cables or conduits from open flame damage.
Industry has long sought better materials for use in cable/conduit construction to protect electrical systems against failures. Fire protection is particularly critical when power supply is necessary for maintenance of controls, for example, in aircraft and control systems in nuclear power plants. The art of heat resistant compositions is replete with numerous organic materials which can withstand moderate temperatures for limited times. Materials which are effective for electric arc and flame protection are generally polymeric materials containing flame and fire retardant components such as halogenated plasticizers and polymers, antimony oxides and organic phosphate esters and chlorides. One of the most commonly used is polyvinyl chloride which is flame retarding. However, the chloride containing materials such as polyvinyl chloride have the disadvantage that the hydrogen chloride, which is evolved on decomposition, combines with water resulting in a highly corrosive reaction with nearby metal components. U.S. Pat. No. 4,273,879 is an illustrative prior effort to provide a material that is more resistant to fire. The active material is an alkali metal silicate and char-forming resins such as phenolic resins, polycarbodiimide and melamine formaldehydes. The binder materials include char-forming polymers such as chloroprene and acrylonitrile elastomers.
Inorganic materials in the form of fiber blankets have also been used to protect electrical systems from the adverse effects of open flames. One such fiber blanket is marketed under the trademark "Kaowool" by Babcock & Wilcox. As recommended, a blanket having a thickness of at least two inches must be used to achieve adequate fire protection. It is believed such thicknesses of fiber unduly restrict the dissipation of resistive heat losses generated within the electrical cable or conduit. This results in a derating of the cable or conduit for the amount of current that can be carried.
Along with the search for better materials has been a search for better ways to apply or affix the materials to protect the electrical cables and conduits from open flame damage. For example, U.S. Pat. No. 4,018,983 discloses a fire protective material which is preshaped into sheaths, boots and the like. The protective material is essentially a chlorinated plastisol into which has been combined intumescing or heat foamable substances, heat resistant fibers such as aramide fibers, fire retardant plasticizers, organic-metallic heat stabilizers and glassy microspheres. In U.S. Pat. No. 4,297,526 a cable is insulated with a thermoplastic material and a filler comprised of a thermoplastic admixture. On exposure to fire, the polymer degradation gases are absorbed by the filler and cause the filler to expand and seal the cable to longitudinal flow of hot gases. As another example, U.S. Pat. No. 4,265,953 discloses insulating wrappings comprising a stressable substrate having intumescent material adhered to the substrate. The intumescent material is expanded after installation to tension the stressable substrate which then becomes a stressed skin which severely limits the expansion of the intumescent material. It is not believed the wrappings would provide adequate protection of an electrical cable or conduit from the heat generated by an open flame.
While many of the known materials function well for their intended purposes, they generally fail to provide the necesssary protection of electrical conduits or cables from direct exposure to fires. When exposed to the heat and flames of a fire, the intumescent materials tend to crack open when expanded in their circular form. Such open cracks expose the conduit or cable to open flame.